I have worked up the final drawings and going with Jazz's approach on the use of 80x40 heavy extrusions
Ditto on using heavy sections, I bought a job lot cheap on ebay a year or two ago and it was a mix of light and heavy, the light is much weaker and only really any good for display cabs.

Perhaps I will have to have HIWIN on the Z and build in some upgrade paths for HIWIN on X and Y in the future if I find the machine needs it.
If you can, go for the profile rails on the z axis as this is always going to be the weakest section, loaded as a cantilever and also eccentric loading from the weight of the z axis.

20mm plate for the gantry sides.
Ok this is only my opinion and I realise that there are lots of machines thathave been built this way and work fine but to me it is a very inefficient way too do it. The gantry sides are the final link in the chain of transmitting force to the ground so really need to be treated as beams/columns. I would go for using 4040 extrusion with 6mm plates either side, probably the same weight but much stiffer. Again I'm not criticising using plate, it just not the way I would do it.